Petroleum refiners produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates, as well as lower boiling hydrocarbonaceous liquids, such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil or heavy fractions thereof. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by fractionation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above 371° C. (700° F.), usually at least about 50% by weight boiling above 371° C. (700° F.). A typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen to yield a product containing a distribution of hydrocarbon products desired by the refiner.
Refiners also subject residual hydrocarbon streams to hydrodesulfurization to produce heavy hydrocarbonaceous compounds having a reduced concentration of sulfur. Residual hydrocarbons contain the heaviest components in a crude oil and a significant portion is non-distillable. Residual hydrocarbon streams are the remainder after the distillate hydrocarbons have been removed or fractionated from a crude oil. A majority of the residual feedstock boils at a temperature greater than about 565° C. (1050° F.). During the desulfurization of residual hydrocarbon feedstocks, a certain amount of distillate hydrocarbons are produced including diesel boiling range hydrocarbons. However, the diesel boiling range hydrocarbons thereby produced typically fail to qualify as ultra-low sulfur diesel because of their relatively high sulfur concentration. Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydroprocessing methods which provide lower costs, more valuable product yields and improved operability.